Hey Laura - please don't think that you'll be way out of your element, as Cascade's training series might be ideal for you; it's planned out so as to ramp up so that you get your miles increased with each ride, riding with a group, learning new routes and I'd recommend going! I'm sure you'll do fine and you'll appreciate the benefits for yourself in helping you train for RSVP. That's a beautiful ride and you'll want to be able to ride it w/o a struggle so you can fully enjoy it.
Try out the CTS and let us know how it goes for you, ok?